Deamidation of a protein improves various functional properties of the protein (Non-patent document 1). Therefore, it is expected that deamidation of a protein expands use of the protein. Among the amino acids that constitute proteins, glutamine and asparagine have an amide group. These are converted into glutamic acid and aspartic acid by deamidation, respectively. By deamidation, the negative charge of a protein is increased, and the isoelectric point of the protein is lowered. The solubility and water dispersibility of the protein are thereby markedly increased. Further, along with an increase of the electrostatic repulsive force, the interaction of proteins, i.e., association property of the proteins, is reduced. It is also known that the tertiary structure of a protein is loosen by deamidation to provide a change of the higher-order structure, so that a hydrophobic region that has been buried in the inside of the protein molecule is exposed on the molecular surface, and therefore the deamidated protein comes to have amphipathy, which results in improvement of the emulsification power, emulsification stability, foamability, and foam stability of the protein.
Techniques for deamidation of proteins are classified into chemical techniques and enzymatic techniques. As the chemical deamidation techniques, many methods based on a mild acid or alkaline treatment have been reported. However, all of these have problems, such as problems that they are based on nonspecific reactions, peptide bonds are also cleaved under the acidic or alkaline condition, unexpected by-products are produced. Further, they also have problems that they require facilities for using chemical substances, and impose significant environmental loads. The enzymatic techniques can overcome such problems of the chemical techniques. As the enzymatic deamidation techniques, there have been reported, for example, methods of using protein glutaminase (Patent documents 1 and 2, Non-patent document 2), methods of using protease (Non-patent documents 3 and 4), a method of using transglutaminase (Non-patent document 5), methods of using peptide glutaminase (Non-patent documents 6 and 7), and so forth.
Among these enzymes, protein glutaminase is the only enzyme that can catalyze deamidation reaction of a high molecular protein without accompanying any side reaction. Use of protease, transglutaminase, and peptide glutaminase has problems since the main reaction catalyzed by protease is cleavage of peptide bonds, the main reaction catalyzed by transglutaminase is crosslinking reaction based on formation of isopeptide bonds between glutamine and lysine, and peptide glutaminase is an enzyme that mainly catalyzes deamidation of decomposed low molecular weight peptides. It is considered that protein glutaminase has high practicality as an enzyme having a high deamidation ability for a high molecular weight protein. There have already been reported findings concerning improvement in functions of wheat proteins, milk proteins (casein and whey proteins), and soybean proteins provided by protein glutaminase (Non-patent documents 8 to 11). In patent documents, there have also been reported findings concerning improvement of qualities of actual foods, for example, yogurt, ice cream, coffee whitener, noodles, meat, and so forth provided by protein glutaminase (Patent documents 3 to 7). However, it is definitely described that protein glutaminase uses a glutamine residue in a protein as a substrate, and does not act on asparagine residue at all (Non-patent documents 2 and 12), and hence, effect of the treatment with this enzyme is limited. In fact, amino acids constituting plant and animal proteins contain a large amount of asparagine, and hence, it is preferable to deamidate not only glutamine, but also asparagine, for obtaining further functional reforming effect by deamidation.
Asparaginase (EC 3.5.1.1) is widely known as an enzyme that catalyzes hydrolysis of asparagine to generate aspartic acid. However, asparaginase is an enzyme that specifically acts on asparagine of the free form, and cannot deamidate an asparagine residue in a peptide or high molecular weight protein. There is also known an enzyme that catalyzes the reaction of deamidating an N-terminus asparagine residue having a free α-amino group is known (Non-patent document 13). However, this enzyme cannot deamidate an asparagine residue in a protein other than N-terminus asparagine residue.
As described above, any enzyme that deamidates an asparagine residue in a protein (except for N-terminus asparagine residue having a free α-amino group) is not known.